Apparatus and method for determining clamping status of semiconductor wafer

ABSTRACT

An apparatus determines how well a semiconductor wafer ( 4 ) is clamped to a support member ( 1 ). The apparatus has at least one ultrasonic transducer ( 2   a,   2   b,   2   c,   2   d ) configured to emit ultrasonic energy ( 3 ) toward an interface between the wafer ( 4 ) and the support member ( 1 ) so that the interface generates echo signals, and a data processing unit ( 11 ) configured to analyze the echo signals to arrive at a determination as to how well the semiconductor wafer ( 4 ) is clamped to the support member ( 1 ) before semiconductor process is started. A first method ensures that a wafer ( 4 ) is securely clamped to a support member before a semiconductor process is started.A second method verifies proper de-clamping of a semiconductor wafer ( 4 ) from a support member ( 1 ) before the semiconductor wafer ( 4 ) is removed from the support member ( 1 ) upon completion of a semiconductor process.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to apparatus and methods forreliably determining when a semiconductor wafer is securely clamped inplace. More specifically, the invention relates to apparatus and methodsthat use ultrasonic techniques to reliably determine whether or not asemiconductor wafer is securely clamped in place on a support member.

[0003] 2. Discussion of the Background

[0004] Plasma processing of silicon wafers to transfer a pattern of anintegrated circuit from the photolithographic mask to the silicon, or todeposit dielectric or conductive films on the substrate, have becomestandard methods in the industry. In conventional plasma processors, thesilicon wafer being processed is held in close proximity to the waferchuck, ordinarily by electrostatic force. This system is quite effectivein holding the wafer securely to the chuck during processing, permittinggood heat transfer between the wafer and the other components in theprocessing system.

[0005] However, all modem manufacturing plasma processors use automaticrobotic systems to load the wafer on and off of the wafer chuck. It isessential, therefore, that the status of the clamping between the waferand the wafer chuck be confirmed after the wafer has been loaded ontothe wafer chuck and before it is removed by the loading arm. Failure ofclamping after loading the wafer onto the wafer chuck will result inpoor quality and uniformity of the process, resulting in poor yields andpoor quality of the finished devices.

[0006] During deposition of metal on a silicon wafer to form conductivepaths that are the electrical interconnects in an integrated circuit,for example, large amounts of electrical energy are delivered to theprocess chamber to transfer the metal ions from the source of metal tothe wafer. As the metal is deposited, energy is delivered to the waferand if the wafer is not cooled, heat may damage the electrical devices.To keep the wafer cool and to promote formation of the desiredmetallurgical compounds on the surface of the wafer, the wafer iselectrostatically clamped to a chilled surface and a gas is introducedbehind the wafer to enhance heat transfer from the wafer to the chilledsurface

[0007] Because the gas behind the wafer is pressurized to enhance heattransfer, if the wafer is not securely clamped before the gas isintroduced behind the wafer, the wafer will float on the cushion of gas,away from its position on the chilled surface. Thus, it is desirable toverify that the wafer is properly clamped before introducing the gasbehind the wafer, and to maintain a secure clamp as long as pressurizedgas is present behind the wafer.

[0008] When the processing is complete, the electrostatic potentialholding the wafer to the wafer chuck is turned off. However, residualelectrostatic charges may inhibit release of the wafer. In this case,when the robotic system attempts to remove the wafer from the waferchuck, the wafer may be broken. This is catastrophic, because not onlyis the valuable wafer lost, it is usually necessary to do a completetear down and clean out of the processing chamber, costing valuable timeand manpower as well as lost production time on the equipment.

[0009] Therefore, it is desirable to continuously monitor the status ofthe clamping between the wafer and wafer chuck.

[0010] One alternative method for detecting the presence of a wafer andwhether the wafer is satisfactorily clamped is by measuring thecapacitance between the wafer and the surface on which the wafer isclamped. The capacitance is measured by injecting a sample RF signalonto an electrode under the wafer and measuring the intensity of thesample frequency RF on a second electrode under the wafer.

[0011] A second alternative method for detecting whether the wafer issatisfactorily clamped is by measuring the flow necessary to maintain apressure of gas under the wafer. By using very small flows to produce avery slight pressure under the wafer, it may be possible to determinethe quality of the clamp of the wafer prior to applying gas pressureunder the wafer. Similarly, this technique may be used to determinewhether the wafer has been de-clamped from the surface. This technique,however, is not useful for monitoring the clamp status during processingof the wafer since it relies on the opportunity to adjust the pressureor flow of gas behind the wafer. Also, variations in the surface of thewafer may result in variations in flow even though the wafer is securelyclamped to the surface.

[0012] U.S. Pat. No. 5,271,274 (Khuri-Yakub et al.) discloses a methodusing ultrasonic acoustic waves to determine the presence and thicknessof films on a substrate. The echo of the ultrasonic wave or the phase ofthe echo is used to measure the thickness of deposited films on asubstrate such as a silicon wafer. U.S. Pat. No. 6,019,000 (Stanke etal.) utilizes ultrasonic acoustic waves to perform in-situ measurementof deposition on reactor chamber members. This system also permits thedetermination of the degree of erosion of chamber members. Both patentsutilize reflection of the ultrasonic waves from the surfaces andinterfaces between the members and films to determine the presence andthickness of the films. However, neither patent solves the problems ofmonitoring the status of the clamping between a wafer and a wafer chuck,and of determining when the wafer has been completely released byelectrostatic forces holding it to the wafer chuck. It is to fulfillthese needs, among others, that the present invention is directed.

SUMMARY OF THE INVENTION

[0013] An important feature of the invention is the use of ultrasonictransducers to determine the status of the clamping of a wafer to awafer chuck. The status of the wafer clamping is determined in real timecontinually during the wafer processing cycle. Thus, mis-processing ofwafers because of improper or incomplete clamping is avoided. Also,since the degree of clamping force is measured by continually monitoringthe intensity of a reflected signal, this measurement is used todetermine the effectiveness of the electrostatic chuck and to timelydetect incipient failure of the clamp.

[0014] Another important feature is the use of ultrasonic transducers todetermine when the wafer has been completely released by theelectrostatic forces holding it to the wafer chuck. The complete releaseof the wafer by the electrostatic chuck is determined, thus permittingthe safe removal of the wafers from the processing chamber by a roboticsystem, without danger of breaking the wafer.

[0015] Other objects, features and advantages of the present inventionwill be apparent to those skilled in the art upon a reading of thisspecification including the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention is better understood by reading the followingDetailed Description of the Preferred Embodiments with reference to theaccompanying drawing figures, in which like reference numerals refer tolike elements throughout, and in which:

[0017]FIG. 1 is a block diagram schematically illustrating an exemplaryapparatus for determining the clamping status of a semiconductor wafer 4on an electrostatic chuck 1;

[0018]FIGS. 2A, 2B and 2C illustrate ultrasonic intensity as a functionof time, for scenarios in which a semiconductor wafer is securelyattached to an electrostatic chuck (FIG. 2A), the wafer is partiallyremoved from the chuck (FIG. 2B), and the wafer is completely removedfrom the chuck (FIG. 2C);

[0019]FIGS. 2D, 2E and 2F illustrate another example of ultrasonicintensity as a function of time, for scenarios in which a semiconductorwafer is securely attached to an electrostatic chuck (FIG. 2D), thewafer is partially removed from the chuck (FIG. 2E), and the wafer iscompletely removed from the chuck (FIG. 2F);

[0020]FIGS. 3A and 3B (which may be collectively referred to herein as“FIG. 3”) are flow charts illustrating exemplary methods of ensuringthat a semiconductor wafer is securely clamped to a chuck before asemiconductor process is started (FIG. 3A), and of ensuring that asemiconductor wafer is properly de-clamped from the chuck before thewafer is mechanically lifted from the chuck after the semiconductorprocess is completed (FIG. 3B); and

[0021]FIG. 4 shows an exemplary method for verifying proper clampingpressure of a wafer on a chuck during a wafer process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] In describing preferred embodiments of the present inventionillustrated in the drawings, specific terminology is employed for thesake of clarity. However, the invention is not intended to be limited tothe specific terminology so selected, and it is to be understood thateach specific element includes all technical equivalents that operate ina similar manner to accomplish a similar purpose.

[0023] The concept of ultrasonics is that if an ultrasonic wave isintroduced into an object, it will be partially reflected back from anyinterface in that object. Therefore, if an ultrasonic transducer isapplied to an object and the resultant reflection images are suitablyprocessed, a considerable amount of information about the object can bedetermined.

[0024] In an exemplary embodiment of the instant invention shown in FIG.1, one or more ultrasonic transducers 2 a, 2 b, 2 c, 2 d (collectivelyreferred to herein as element 2) are placed on the backside of a waferchuck 1. For example, the ultrasonic transducers 2 can be eitherpiezoelectric or electro-mechanical/acoustic transducers (EMAT) and canbe attached to the wafer chuck 1 by any suitable means. By observing thereflection patterns of ultrasonic waves (schematically indicated byreference numeral 3), the status of the clamping of a wafer 4 to chuck 1is determined.

[0025]FIG. 1 illustrates a transducer interface unit 10, which allows adata processing unit 11 to control the application of electrical signalsto the various transducers 2 a, 2 b, 2 c, 2 d and so forth, so that thetransducers propagate ultrasonic waves into chuck 1. Transducerinterface unit 10 also receives the return signals indicative of thereflected ultrasonic waves, and reports the magnitude of these signalsas a function of time to data processing unit 11.

[0026] Data processing unit 11 analyzes the return signals to formdecisions as to the clamping status of the wafer on the chuck. Anyinterpretive reports that the data processing unit may generate, as wellas the raw return signals, may be presented to users on display 12.

[0027] Further, when an alarm condition is present, such as when thewafer process should be stopped due to improper clamping of the wafer orincomplete de-clamping when it is desired to mechanically remove thewafer, then the data processing unit may cause an audible or other typeof alarm 13 to be activated. The user may control the semiconductorprocess accordingly, or, in an alternative embodiment, the dataprocessing unit 11 may control the semiconductor process controller 14directly.

[0028] Transducer interface 10, data processing unit 11, display 12 andalarm 13 may be of conventional design, and are commercially availableor readily designed by those skilled in the art. For example, dataprocessing unit 11 may be any suitable general purpose or specialpurpose computer that is capable of executing programs implementing themethods described later in this specification. Semiconductor processcontroller 14 is not an element of the present invention as such, but isunderstood to already be provided as part of the semiconductor processequipment with which the invention may be practiced. Accordingly,additional details of these elements need not be provided herein.

[0029] Referring again to FIG. 1, when wafer 4 is intimately clamped towafer chuck 1, an ultrasonic wave 3 propagates across the interfacescreated therein. As shown in the plot of ultrasonic wave intensity as afunction of time shown in FIG. 2A, first and second reflections (echoes)21, 22 are generated. The first and second reflections occur at times t₁and t₂, respectively.

[0030] First reflection 21 is from the interface between wafer 4 andchuck 1, and second reflection 22 is from the interface on the far sideof wafer 4. According to one embodiment of the invention, the relativeintensities of the two reflections 21, 22 in FIG. 2A determine thatwafer 4 is securely clamped to chuck 1. For example, secure clamping isconfirmed by the fact that the intensity of second peak 22 at time t₂ isgreater than a first intensity threshold Th₁.

[0031] The invention recognizes that, if plural ultrasonic transducers 2are employed, the clamping status of wafer 4 as a function of positionaround chuck 1 may be determined. Accordingly, plural transducers 2 a, 2b, 2 c, 2 d and so forth, are provided, and may function in the samemanner as described above for a single transducer, except that theultrasonic waves are launched in a time-division multiplexed manner soas to avoid confusion of echoes from neighboring transducers.

[0032] When wafer 4 is to be de-chucked, electrostatic potential isremoved. Partial (incomplete) release of the wafer 4 from the waferchuck 1 is shown from the ultrasonic reflection pattern as shown in FIG.2B. As wafer 4 is released from chuck 1, there is reduced propagation ofthe ultrasonic wave 3 across the interface. In that case, therecontinues to be a single reflection 31 from the top of the chuck 1, butthe second reflection 32 from the upper surface of the wafer 4 is ofdiminishing intensity after the electrostatic potential is removed. Thisis confirmed when the intensity of second peak 32 at time t₂ is lessthan first intensity threshold Th₁ (mentioned above) but greater than asecond intensity threshold Th₂. Second intensity threshold Th₂ is lessthan first intensity threshold Th₁.

[0033] When wafer 4 is completely removed from the surface of chuck 1(as distinguished from the partial release discussed immediately above),the second reflection from the upper surface of wafer 4 can disappearentirely. For example, the disappearance of the second reflection canconfirm that the wafer 4 is no longer present on the surface of chuck 1.This waveform is shown in FIG. 2C, which shows only a first reflection41. Alternately, there is no second intensity peak at time t₂ thatexceeds second intensity threshold Th₂.

[0034] In plural-transducer embodiments such as that illustrated in FIG.1, plural transducers 2 ensure that wafer 4 is completely released atvarious locations around the chuck, and that it is safe for the roboticsystem to remove the wafer.

[0035] Threshold values Th₁ and Th₂ can be determined as absolutenumbers, by direct experimentation with a particular setup.Alternatively, to reduce such experimentation, threshold values Th₁ andTh₂ can be determined as respective fractions of first peak intensityvalue at time t₁.

[0036] In an alternative embodiment shown in FIGS. 2D-2F, the clampingstatus can be determined from the time difference between tworeflections. For example, the time difference between peak 51 attime t₁and peak 52 at time t₂shown in FIG. 2D can be less than a firstthreshold difference Th₃, indicating secure clamping; the timedifference between peak 61 at time t₁ and peak 62 at t₂ shown in FIG. 2Ecan be greater than the first threshold difference Th₃ and less than asecond threshold difference Th₄, indicating partial clamping; and thetime difference between peak 71 at time t₁ and peak 72 at time t₂ shownin FIG. 2F can be greater than the second threshold difference Th₄,indicating de-clamping.

[0037] Time dependent thresholds Th₃ and Th₄ can be determined asabsolute numbers, by direct experimentation with a particular setup.Alternatively, to reduce such experimentation, time dependent thresholdsTh₃ and Th₄ can be determined as respective time delays added to an echosignal occurring at a shorter time These echo signals can originate fromreflections from different material layers in chuck 1 or from reflectionfrom the space between chuck 1 and wafer 4 (first intensity peak at t₁).

[0038] In yet another embodiment, the clamping status can be determinedusing echo signal (not shown) corresponding to reflection from the lowersurface of the wafer 4. This reflection is expected to substantiallyoverlap with first intensity peak at t₁ when the wafer 4 is securelyclamped to chuck 1. When the wafer 4 is partially or completely releasedfrom the surface of chuck 1, the peak corresponding to reflection fromthe bottom of wafer 4 will occur at a time that is greater than thefirst intensity peak at time t₁.

[0039]FIGS. 3A and 3B (which may collectively be referred to herein asFIG. 3) show exemplary methods of operation provided by the presentinvention. FIG. 3A shows an exemplary method by which secure clamping ofa wafer to a chuck is verified, before the wafer is processed. FIG. 3Bshows an exemplary method for verifying proper de-clamping of the waferfrom the chuck before removal of the wafer upon completion of the waferprocess.

[0040] Referring to FIG. 3A, step 301 starts the method. The robotic armloads the wafer into the chamber in step 302, and sets the wafer on thelift pins in step 304 that were raised in step 303. The wafer is thenlowered to the chuck in step 305, and electrostatic clamping force isapplied in step 306.

[0041] Step 307 determines the status of the wafer clamping usingultrasonic techniques described above. If the clamp is not good, thesequence proceeds to step 309 in which the process is stopped andproceeds to step 310 in which an alarm is sent to the operator. If theclamp is determined to be good, the sequence proceeds to turn thebackside gas on in step 311 and to process the wafer in step 312.Alternately, steps 307-310 can be run after step 311 and also after step312. FIG. 3B shows an exemplary method for verifying proper de-clampingof a wafer from a chuck before removal of the wafer upon completion of awafer process.

[0042] Referring to FIG. 3B, when the process is complete as determinedin step 320, the backside gas is turned off in step 321, and theelectrostatic clamping potential is turned off in step 322. Step 324determines if the wafer has been properly de-clamped in accordance withthe principles described above. If the wafer has not been properlyde-clamped, the operation is stopped at step 325 and an alarm is sent tothe operator in step 326. If the wafer has been successfully de-clamped,it may be safely raised with the lift pins in step 327 and then beremoved from the processing chamber by the robotic arm in step 328. Atthis time, the next wafer sequence may begin (step 301 in FIG. 4A), asshown in step 329.

[0043] In an alternate embodiment, a procedure can be performed toensure that the proper clamping pressure is applied. Clamping the wafertoo securely causes excessive numbers of particles to be generated onthe back of the wafer. Confirming that the wafer is adequately clampedallows one to limit the clamping force to the amount required to clampthe wafer. In this way, one may avoid using force in excess of therequired force to clamp the wafer and thereby, avoid creatingtroublesome particles. Applying this method in real time may allow oneto adjust the clamp force (by increasing the electrostatic voltage) asrequired to keep the wafer securely in place as the backside gaspressure varies.

[0044]FIG. 4 shows an exemplary method for verifying proper clampingpressure of a wafer on a chuck during a wafer process. Procedure 400starts in step 405.

[0045] In step 410, the wafer is placed on the chuck.

[0046] In step 415, voltage is applied to clamp the wafer to the chuck.Clamp voltage can generate force in excess of the force necessary tohold the wafer. Typically equipment is configured for worst caseconditions and then every condition is handled as though it is worstcase.

[0047] In step 420, a query is performed to determine when the clampingvoltage reaches a first clamping threshold. A sufficient voltage isapplied to clamp the wafer, but only to reach the first “clamping”threshold. When the first clamping threshold is exceeded, procedure 400continues to step 425. When the first clamping threshold is notexceeded, procedure 400 branches back to step 415.

[0048] In step 425, the backside gas is turned on and backside of waferis pressurized.

[0049] In step 430, the clamping voltage is increased.

[0050] In step 435, a query is performed to determine when the clampingvoltage reaches a second clamping threshold. A sufficient voltage isapplied to clamp the wafer, but only to reach the second “clamping”threshold. Desirably, the second clamping threshold is slightlydifferent than the first clamping threshold. When the second clampingthreshold is exceeded, procedure 400 continues to step 440. When thesecond clamping threshold is not exceeded, procedure 400 branches backto step 430.

[0051] In step 440, a query is performed to determine if the clampingvoltage is correct. The “correct value” is established by providing anoperational range around the second clamping threshold voltage.Desirably, the operational range provides for changes in the backsidegas. When the clamping voltage is correct, procedure 400 continues tostep 450. When the clamping voltage is not correct, procedure 400branches to step 445 where the clamping voltage is adjusted.

[0052] In step 450, a query is performed to determine if the process iscompleted. Desirably, the clamping voltage is monitored during a processto account for changes in the backside gas pressure. When the process iscompleted, procedure 400 continues to step 455. When the process is notcompleted, procedure 400 branches back to step 440.

[0053] In step 455, the backside gas is turned-off.

[0054] In step 460, the clamping voltage is maintained as the backsidegas pressure changes.

[0055] In step 465, the clamping voltage is turned off. In step 470, thewafer is removed and procedure 400 ends in step 475.

[0056] Modifications and variations of the above-described embodimentsof the present invention are possible, as appreciated by those skilledin the art in light of the above teachings. For example, the number andarrangement of transducers on the chuck, and the particularimplementations of elements such as the transducers, interface and dataprocessing unit, may be varied while remaining within the scope of thepresent invention. It is therefore to be understood that, within thescope of the appended claims and their equivalents, the invention may bepracticed otherwise than as specifically described.

1. An apparatus for determining how well a semiconductor wafer isclamped to a support member, the apparatus comprising: an ultrasonictransducer configured to emit ultrasonic energy toward an interfacebetween the semiconductor wafer and the support member, so that echosignals are generated; and a data processing unit configured to analyzethe echo signals to arrive at a determination as to how well thesemiconductor wafer is clamped to the support member.
 2. The apparatusof claim 1, wherein: the echo signals include a first intensity peakoccurring at a first time; and the data processing unit is configured toanalyze the echo signals for a second intensity peak occurring at asecond time after the first time, and to determine that thesemiconductor wafer is securely clamped to the support member if thesecond intensity peak is larger than a first threshold intensity.
 3. Theapparatus of claim 2, wherein: the first threshold intensity isdetermined as a fraction of the first intensity peak.
 4. The apparatusof claim 1, wherein: the echo signals include a first intensity peakoccurring at a first time; and the data processing unit is configured toanalyze the echo signals for a second intensity peak occurring at asecond time after the first time, and to determine that thesemiconductor wafer is completely de-clamped from the support member ifthe second intensity peak is smaller than a second threshold intensity.5. The apparatus of claim 4, wherein: the second threshold intensity isdetermined as a fraction of the first intensity peak.
 6. The apparatusof claim 1, wherein: the echo signals include a first intensity peakoccurring at a first time; and the data processing unit is configured toanalyze the echo signals for a second intensity peak occurring at asecond time after the first time, and to determine that semiconductorwafer is neither securely clamped to the support member nor safelyde-clamped from the support member if the second intensity peak isbetween a first threshold intensity and a second threshold intensity. 7.The apparatus of claim 6, wherein: the first threshold intensity and thesecond threshold intensity are determined as respective fractions of thefirst intensity peak.
 8. The apparatus of claim 1, wherein: the supportmember is an electrostatic chuck that clamps the semiconductor wafer byapplication of electrostatic potential.
 9. The apparatus of claim 1,wherein: the data processing unit is configured to present a display toa user indicating its determination as to how well the semiconductorwafer is clamped to the support member.
 10. The apparatus of claim 1,wherein: the data processing unit is configured to send a signal to asemiconductor process controller, indicating the data processing unit'sdetermination as to how well the semiconductor wafer is clamped to thesupport member.
 11. The apparatus of claim 1, further comprising:additional ultrasonic transducers configured to emit additionalultrasonic energy toward the interface between the semiconductor waferand the support member, so that additional echo signals are generatedallowing the data processing unit to analyze the additional echo signalsfrom plural additional sites on the support member to arrive at thedetermination as to how well the semiconductor wafer is clamped to thesupport member.
 12. The apparatus of claim 1, wherein the echo signalsinclude a first intensity peak occurring at a first time; and the dataprocessing unit is configured to analyze the echo signals for a secondintensity peak occurring at a second time after the first time, and todetermine that the semiconductor wafer is securely clamped to thesupport member if the time difference between the first intensity peakand the second intensity peak is less than a first threshold difference.13. The apparatus of claim 12, wherein: the first threshold differenceis determined as a time delay added to a first reflection.
 14. Theapparatus of claim 12, wherein: the first threshold difference isdetermined as a time delay added to the first time.
 15. The apparatus ofclaim 1, wherein: the echo signals include a first intensity peakoccurring at a first time; and the data processing unit is configured toanalyze the echo signals for a second intensity peak occurring at asecond time after the first time, and to determine that thesemiconductor wafer is completely de-clamped from the support member ifthe time difference between the first intensity peak and the secondintensity peak is greater than a second threshold difference.
 16. Theapparatus of claim 15, wherein: the second threshold difference isdetermined as a time delay added to a first reflection.
 17. Theapparatus of claim 15, wherein: the second threshold difference isdetermined as a time delay added to the first time.
 18. The apparatus ofclaim 1, wherein: the echo signals include a first intensity peakoccurring at a first time; and the data processing unit is configured toanalyze the echo signals for a second intensity peak occurring at asecond time after the first time, and to determine that semiconductorwafer is neither securely clamped to the support member nor safelyde-clamped from the support member if the second intensity peak occursbetween a first threshold difference and a second threshold difference.19. The apparatus of claim 18, wherein: the first threshold differenceand the second threshold difference are determined as time delays addedto a first reflection.
 20. The apparatus of claim 18, wherein: the firstthreshold difference and the second threshold difference are determinedas time delays added to the first time.
 21. A method for determining howwell a semiconductor wafer is clamped to a support member, the methodcomprising: emitting ultrasonic energy toward an interface between thesemiconductor wafer and the support member, so that echo signals aregenerated; and analyzing the echo signals to determine how well thesemiconductor wafer is clamped to the support member.
 22. The method ofclaim 21, wherein: the echo signals include a first intensity peakoccurring at a first time; and the analyzing step includes analyzing theecho signals for a second intensity peak occurring at a second timeafter the first time, and determining that the semiconductor wafer issecurely clamped to the support member if the second intensity peak islarger than a first threshold intensity.
 23. The method of claim 22,wherein: the first threshold intensity is determined as a fraction ofthe first intensity peak.
 24. The method of claim 21, wherein: the echosignals include a first intensity peak occurring at a first time; andthe analyzing step includes analyzing the echo signals for a secondintensity peak occurring at a second time after the first time, anddetermining that the semiconductor wafer is completely de-clamped fromthe support member if the second intensity peak is smaller than a secondthreshold intensity.
 25. The method of claim 24, wherein: the secondthreshold intensity is determined as a fraction of the first intensitypeak.
 26. The method of claim 21, wherein: the echo signals include afirst intensity peak occurring at a first time; and the analyzing stepincludes analyzing the echo signals for a second intensity peakoccurring at a second time after the first time, and determining thatsemiconductor wafer is neither securely clamped to the support membernor safely de-clamped from the support member if the second intensitypeak is between a first threshold intensity and a second thresholdintensity.
 27. The method of claim 26, wherein: the first thresholdintensity and the second threshold intensity are determined asrespective fractions of the first intensity peak.
 28. The method ofclaim 21, wherein: the support member is an electrostatic chuck thatclamps the semiconductor wafer by application of electrostaticpotential.
 29. The method of claim 21, further comprising: presenting adisplay to a user indicating the determination as to how well thesemiconductor wafer is clamped to the support member.
 30. The method ofclaim 21, further comprising: sending a signal to a semiconductorprocess controller, indicating the determination as to how well thesemiconductor wafer is clamped to the support member.
 31. The method ofclaim 21, further comprising: emitting additional ultrasonic energytoward the interface between the semiconductor wafer and the supportmember, so that additional echo signals are generated allowing theanalyzing step to analyze the additional echo signals from pluraladditional sites on the support member to arrive at the determination asto how well the semiconductor wafer is clamped to the support member.32. The method of claim 21, wherein: the echo signals include a firstintensity peak occurring at a first time; and the analyzing stepincludes analyzing the echo signals for a second intensity peakoccurring at a second time after the first time, and determining thatthe semiconductor wafer is securely clamped to the support member if thetime difference between the first intensity peak and the secondintensity peak is less than a first threshold difference.
 33. The methodof claim 32, wherein: the first threshold difference is determined as atime delay added to a first reflection.
 34. The method of claim 32,wherein: the first threshold difference is determined as a time delayadded to the first time.
 35. The method of claim 21, wherein: the echosignals include a first intensity peak occurring at a first time; andthe analyzing step includes analyzing the echo signals for a secondintensity peak occurring at a second time after the first time, anddetermining that the semiconductor wafer is completely de-clamped fromthe support member if the time difference between the first intensitypeak and the second intensity peak is greater than a second thresholddifference.
 36. The method of claim 35, wherein: the second thresholddifference is determined as a time delay added to a first reflection.37. The method of claim 35, wherein: the second threshold difference isdetermined as a time delay added to the first time.
 38. The method ofclaim 21, wherein: the echo signals include a first intensity peakoccurring at a first time; and the analyzing step includes analyzing theecho signals for a second intensity peak occurring at a second timeafter the first time, and determining that semiconductor wafer isneither securely clamped to the support member nor safely de-clampedfrom the support member if the second intensity peak occurs between afirst threshold difference and a second threshold difference.
 39. Themethod of claim 38, wherein: the first threshold difference and thesecond threshold difference are determined as time delays added to afirst reflection.
 40. The method of claim 38, wherein: the firstthreshold time and the second threshold time are determined as timedelays added to the first time.
 41. A method of ensuring that asemiconductor wafer is securely clamped to a support member before asemiconductor process is started, the method comprising: a) placing thesemiconductor wafer in proximity to the support member; b) applying aforce that tends to clamp the semiconductor wafer to the support member;c) measuring a degree by which the semiconductor wafer is clamped to thesupport member, to arrive at a determination of whether thesemiconductor wafer is securely clamped to the support member, whereinthe measuring step includes: 1) emitting ultrasonic energy toward aninterface between the semiconductor wafer and the support member, sothat echo signals are generated; and 2) analyzing the echo signals todetermine whether the semiconductor wafer is securely clamped to thesupport member; and d) based on the determination, either continuingwith the semiconductor process or aborting the semiconductor process.42. The method of claim 41, wherein the force applying step includes:applying electrostatic potential to the support member.
 43. The methodof claim 41, wherein the aborting step includes: communicating an alarmto an operator, indicating that the semiconductor wafer is not securelyclamped to the support member.
 44. The method of claim 41, wherein: theecho signals include a first intensity peak occurring at a first time;and the analyzing step includes analyzing the echo signals for a secondintensity peak occurring at a second time after the first time, anddetermining that the semiconductor wafer is securely clamped to thesupport member if the second intensity peak is larger than a thresholdintensity.
 45. The method of claim 44, wherein: the threshold intensityis determined as a fraction of the first intensity peak.
 46. The methodof claim 41, further comprising: presenting a display to a userindicating the determination as to whether the semiconductor wafer issecurely clamped to the support member.
 47. The method of claim 41,further comprising: sending a signal to a semiconductor processcontroller, indicating the determination as to whether the semiconductorwafer is securely clamped to the support member.
 48. The method of claim41, further comprising: emitting additional ultrasonic energy toward theinterface between the semiconductor wafer and the support member, sothat additional echo signals are generated allowing the analyzing stepto analyze the additional echo signals from additional sites on thesupport member to arrive at the determination as to whether thesemiconductor wafer is securely clamped to the support member.
 49. Themethod of claim 41, wherein the force applying step includes: applyingelectrostatic potential to the support member; comparing the appliedelectrostatic potential to a threshold; and adjusting the appliedelectrostatic potential if the threshold is not exceeded.
 50. The methodof claim 41, wherein: the echo signals include a first intensity peakoccurring at a first time; and the analyzing step includes analyzing theecho signals for a second intensity peak occurring at a second timeafter the first time, and determining that the semiconductor wafer issecurely clamped to the support member if the time difference betweenthe first intensity peak and the second intensity peak is less than afirst threshold difference.
 51. The method of claim 50, wherein: thefirst threshold difference is determined as a time delay added to afirst reflection.
 52. The method of claim 50, wherein: the firstthreshold difference is determined as a time delay added to the firsttime.
 53. A method for verifying proper de-clamping of a semiconductorwafer from a support member before the semiconductor wafer is removedfrom the support member upon completion of a semiconductor process, themethod comprising: a) releasing a force that tends to clamp thesemiconductor wafer to the support member; b) measuring a degree bywhich the semiconductor wafer is clamped to the support member, toarrive at a determination of whether the semiconductor wafer is properlyde-clamped from the support member, wherein the measuring stepincludes: 1) emitting ultrasonic energy toward an interface between thesemiconductor wafer and the support member, so that the interfacegenerates echo signals; and 2) analyzing the echo signals to determinewhether the semiconductor wafer is properly de-clamped from the supportmember; and c) if the determination indicates that the semiconductorwafer is properly de-clamped from the support member, physicallyremoving the semiconductor wafer from the support member and continuingthe semiconductor process on a subsequent semiconductor wafer.
 54. Themethod of claim 53, wherein the force releasing step includes: removingelectrostatic potential that had been applied to the support memberduring the semiconductor process.
 55. The method of claim 53, whereinthe aborting step includes: communicating an alarm to an operator,indicating that the semiconductor wafer is not properly de-clamped fromthe support member.
 56. The method of claim 53, wherein: the echosignals include a first intensity peak occurring at a first time; andthe analyzing step includes analyzing the echo signals for a secondintensity peak occurring at a second time after the first time, anddetermining that the semiconductor wafer is properly de-clamped from thesupport member if the second intensity peak is smaller than a thresholdintensity.
 57. The method of claim 56, wherein: the threshold intensityis determined as a fraction of the first intensity peak.
 58. The methodof claim 53, wherein: the echo signals include a first intensity peakoccurring at a first time; and the analyzing step includes analyzing theecho signals for a second intensity peak occurring at a second timeafter the first time, and determining that the semiconductor wafer isproperly de-clamped from the support member if the time differencebetween the first intensity peak and the second intensity peak isgreater than a second threshold difference.
 59. The method of claim 58,wherein: the second threshold difference is determined as a time delayadded to a first reflection.
 60. The method of claim 58, wherein: thesecond threshold difference is determined as a time delay added to thefirst time.
 61. The method of claim 53, further comprising: presenting adisplay to a user indicating the determination as to whether thesemiconductor wafer is properly de-clamped to the support member. 62.The method of claim 53, further comprising: sending a signal to asemiconductor process controller, indicating the determination as towhether the semiconductor wafer is properly de-clamped from the supportmember.
 63. The method of claim 53, further comprising: emittingadditional ultrasonic energy toward the interface between thesemiconductor wafer and the support member, so that additional echosignals are generated allowing the analyzing step to analyze theadditional echo signals from additional sites on the support member toarrive at the determination as to whether the semiconductor wafer isproperly de-clamped from the support member.